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Appendix II.

But whatever machine may be adopted, there are some general
rules that apply to every case, though more particularly if the
angle be great or the pitch large, for then any deviation from rule
is more apparent. Imagine a screw A B, having, say, four threads
wrapped round its elementary cylinder. The combination of cut
and feed will cause the work to travel under the tool in the
direction K j, and the tool front must therefore be set normal to
this line. The section at H will not shew the real shape or width
of tool, but that at j, which is taken across the line CD. The
comparison of the two may be shewn by a diagram : thus, if ab
be the true pitch, over four threads, measured axially, and d E the
outside circumference, cd will be the pitch measured normal to
the threads. By setting out the threads on a , and projecting
them on c d^ the true width of tool at thread top may be found,

y. 768.

as at Ci d?i. Again, by making d e equal to circumference at thread
bottom, we have fd as the normal pitch upon which the threads
are to be again projected from a &, giving f2 d2, the width of
tool at thread bottom. Finally, the curvature of tool point
will be that due to the ellipse obtained as section on line c D.
The same rules apply to broad traversing tools for plain lathe-
work, so far as curvature of tool and angle of path is concerned.

 jP. 226. Incorrect Taper-turning.  When cylindrical
work only is being turned, the correctness of the cylinder is not
in the least marred by the height at which the tool is set,
though the beauty of the surface may be very much affected.
But the turning of a tapered surface or cone requires special
"care in this respect, and it is of the highest importance that